If sardine populations don't recover soon, experts warn, the West Coast's marine mammals, seabirds and fishermen could suffer for years.

The sardine fishing boat Eileen motored slowly through moonlit waters from San Pedro to Santa Catalina Island, its weary-eyed captain growing more desperate as the night wore on. After 12 hours and $1,000 worth of fuel, Corbin Hanson and his crew returned to port without a single fish.

"Tonight's pretty reflective of how things have been going," Hanson said. "Not very well."

To blame is the biggest sardine crash in generations, which has made schools of the small, silvery fish a rarity on the West Coast. The decline has prompted steep cuts in the amount fishermen are allowed to catch, and scientists say the effects are probably radiating throughout the ecosystem, starving brown pelicans, sea lions and other predators that rely on the oily, energy-rich fish for food.

If sardines don't recover soon, experts warn, the West Coast's marine mammals, seabirds and fishermen could suffer for years.

The reason for the drop is unclear. Sardine populations are famously volatile, but the decline is the steepest since the collapse of the sardine fishery in the mid-20th century. And their numbers are projected to keep sliding.

One factor is a naturally occurring climate cycle known as the Pacific Decadal Oscillation, which in recent years has brought cold, nutrient-rich water to the West Coast. While those conditions have brought a boom in some species, such as market squid, they have repelled sardines.

If nature is responsible for the decline, history shows the fish will bounce back when ocean conditions improve. But without a full understanding of the causes, the crash is raising alarm.

An assessment last fall found the population had dropped 72% since its last peak in 2006. Spawning has taken a dive too.

In November, federal fishery managers slashed harvest limits by more than two-thirds, but some environmental groups have argued the catch should be halted outright.

"We shouldn't be harvesting sardines any time the population is this low," said Geoff Shester, California program director for the conservation group Oceana, which contends that continuing to fish for them could speed their decline and arrest any recovery.

The Pacific sardine is the ocean's quintessential boom-bust fish. It is short-lived and prolific, and its numbers are wildly unpredictable, surging up and down in decades-long cycles in response to natural shifts in the ocean environment. When conditions are poor, sardine populations plunge. When seas are favorable, they flourish in massive schools.

It was one of those seemingly inexhaustible swells that propelled California's sardine fishery to a zenith in the 1940s. Aggressive pursuit of the species transformed Monterey into one of the world's top fishing ports.

And then it collapsed.

By mid-century sardines had practically vanished, and in the 1960s California established a moratorium on sardine fishing that lasted 18 years. The population rebounded in the 1980s and fishing resumed, but never at the level of its heyday.

Since the 1940s scientists have debated how much of the collapse was caused by ocean conditions and how much by overfishing. Now, researchers are posing the same question.

Separate sardine populations off Japan, Peru and Chile fluctuate in the same 50- to 70-year climate cycle but have been more heavily exploited, Vetter said. West Coast sardines are considered one of the most cautiously fished stocks in the world, a practice that could explain why their latest rebound lasted as long as it did. The West Coast's last sardine decline began in 1999, but the population shot back up by the mid-2000s.

In recent years scientists have gained a deeper understanding of sardines' value as "forage fish," small but nutrition-packed species such as herring and market squid that form the core of the ocean food web, funneling energy upward by eating tiny plankton and being preyed on by big fish, seabirds, seals and whales.

Now, they say, there is evidence some ocean predators are starving without sardines. Scarcity of prey is the leading theory behind the 1,600 malnourished sea lion pups that washed up along beaches from Santa Barbara to San Diego in early 2013, said Sharon Melin, a wildlife biologist at the National Marine Fisheries Service.

Melin's research indicates that nursing sea lion mothers could not find fatty sardines, so they fed on less nutritious market squid, rockfish and hake and produced less milk for their young in 2012. The following year their pups showed up on the coast in overwhelming numbers, stranded and emaciated.

"We are likely to see more local events like this if sardines disappear or redistribute along the coast and into deeper water," said Selina Heppell, a fisheries ecologist at Oregon State University.